The achievement will unlock at the end of the “Volga” level if duke survives.
In order to have Duke survive (and unlock the Duke achievement), you’ll have to maintain a good relationship with the locals. Morale has to be either neutral or positive. This means you can’t kill any cultists. It’s OK to knock them out, but killing them will lower the morale, which means Duke might die. So just avoid killing them altogether. Bandits, on the other hand, are fair game, so don’t hold back there.
Credit:
https://www.gosunoob.com/metro-exodus/duke-damir-alyosha-how...